1. Field of the Invention
The present invention generally relates to optical switches and, in particular, to an optical switch based on total internal reflection that controls fluid pressure in the switch to improve switching characteristics.
2. Related Art
Some internally reflective optical switches change states by forming a bubble in a liquid that is located at the intersection of various waveguide segments. For example, U.S. Pat. No. 5,699,462 entitled "Total Internal Reflection Optical Switches Employing Thermal Activation," which is incorporated by reference, describes an optical switch that uses bubbles to change states.
As shown by FIG. 1, the switch 15 described by the foregoing patent has segments 22-25 of core material surrounded by cladding material 27. Segments 22 and 23 are separated from segments 24 and 25 by a trench 32, which is filled with a liquid 34 (FIG. 2). The index of refraction of the liquid 34 is close to or the same as the index of refraction of the segments 22-24. Therefore, in a first state of switch 15, an optical signal passing through segment 22 is not substantially reflected or refracted when it reaches the trench 32. Instead, the optical signal from segment 22 passes through the liquid 34 and then into segment 24.
The trench 32 also includes a heating device 35 (FIG. 2) located on a substrate 38 that may be used to switch the state of the switch 15. The heating device 35 includes control circuitry for selectively increasing or decreasing the amount of heat generated by the heating device 35. To switch the state of the switch 15, the temperature of the heating device 35 is increased until the temperature of the heating device 35 exceeds the boiling point of the liquid 34, thereby causing a bubble 41 to form in the liquid 34, as shown by FIG. 3. The bubble 41 has an index of refraction substantially different than the index of refraction for the liquid 34 and the segments 22-25, and the bubble 41 extends from segment 22 to segment 24. Therefore, an optical signal passing through segment 22 is reflected at the interface of the segment 22 and the bubble 41. Consequently, an optical signal transmitted by segment 22 is reflected at the boundary between the segment 22 and the bubble 41 and travels along segment 23 instead of segment 24.
To place the switch 15 back into its original state, the temperature of the heating device 35 is decreased until the bubble 41 collapses. In other words, the temperature of the heating device 35 is decreased to or below the boiling point of the liquid 34. Once the bubble 41 collapses, the optical signals traveling along segment 22 are no longer reflected at the end of segment 22, and the optical signals, therefore, pass into segment 24 instead of segment 23.
However, a problem with the switch 15 occurs when a bubble 42 (FIG. 4) inadvertently forms in the trench 32. Under certain conditions, an inadvertent bubble 42 forms in the trench 32 even though the heating device 35 is below the boiling point of the liquid 34. In this condition, signals traveling along segment 22 are reflected toward segment 23 regardless of the temperature of the heating device 35.
Thus, an unaddressed need exists in the industry for a device and method for controlling the formation of bubbles in optical switches such that the bubbles do not inadvertently form in the trench and disrupt the operation of the switch.